How does this calculator decide Pass or Fail?

Pass/Fail compares the rated NC to an ASHRAE HVAC Applications screening maximum for the selected space (e.g. Control Room NC-35, Office NC-30, Equipment Room NC-50, Workshop NC-55).

Is estimated dBA exact?

No. The tool reports a screening estimate dBA ≈ NC + 5…8 dB. Use a calibrated A-weighted measurement for compliance documentation.

When should I use RC or NCB instead of NC?

For low-frequency rumble or tonal noise isolation, perform RC (Room Criterion) or NCB (Balanced Noise Criterion) analysis — NC alone can miss those complaint modes.

Noise Criterion Rating — Executive Office · NC-30 | FieldEngineersKit

1. Formula & conversion factors

ANSI/ASA S12.2 Criteria for Evaluating Room Noise (Noise Criterion curves) · ASHRAE Handbook — HVAC Applications (recommended NC by space type)

L_p(f_i) ≤ L_NC(f_i) → NC

dBA ≈ NC + 5…8

Octave bands f_i ∈ {63, 125, 250, 500, 1000, 2000, 4000, 8000} Hz · NC = lowest clear NC-15…65 curve (ANSI/ASA S12.2 tangent) · ASHRAE space Pass/Fail

Hero NC · Pass/FailCode ANSI S12.2Scope NC-15…65Space ASHRAE HVAC

Field screening using the ANSI/ASA S12.2 octave-band tangent method. Rated NC is the lowest standard curve (NC-15 to NC-65) that is nowhere exceeded by measured L_p. Pass/Fail uses ASHRAE HVAC Applications recommended maxima by space type. Estimated A-weighted level is a coarse NC+5…8 rule — not a substitute for calibrated dBA measurement. For rumble or tonal complaints, use RC or NCB analysis.

L_p(f_i) (Octave-band SPL) — Sound pressure level in each standard octave band (dB).
L_NC(f_i) (NC curve limit) — Published NC-15…65 octave-band limit at frequency f_i.
NC (Noise Criterion rating) — Lowest NC curve that clears every band (tangent method).
dBA est. (A-weighted estimate) — Screening: dBA ≈ NC + 5 to 8 dB.

2. Quick reference lookup table

ANSI/ASA S12.2 NC curve octave-band limits (dB) — tangent screening table
NC	63 Hz	125 Hz	250 Hz	500 Hz	1 kHz	2 kHz	4 kHz	8 kHz
NC-15	47	36	29	22	17	14	12	11
NC-20	51	40	33	26	22	19	17	16
NC-25	54	44	37	31	27	24	22	20
NC-30	57	48	41	35	31	29	28	27
NC-35	60	52	45	40	36	34	33	32
NC-40	64	56	50	45	41	39	38	37
NC-45	67	60	54	49	46	44	43	42
NC-50	71	64	58	54	51	49	48	47
NC-55	74	67	62	58	56	54	53	52
NC-60	77	71	67	63	61	59	58	57
NC-65	80	75	71	68	66	64	63	62

Values from the ANSI/ASA S12.2 NC curve family used for FEK tangent screening. Verify against the current published standard for final HVAC design.

3. How to use this calculator

1
Select space type
Choose Control Room, Executive Office, Equipment Room, or Workshop to set the ASHRAE max NC for Pass/Fail.
2
Enter octave-band SPL
Input L_p (dB) for 63 Hz through 8000 Hz from a sound-level meter or HVAC report.
3
Read NC hero and compliance
Hero shows NC-XX and Pass/Fail versus the selected space limit.
4
Review tangent band and chart
Use the spectrum vs NC curve chart and octave-band rows to see which band governs the rating.

4. Engineering FAQ

An NC rating is the lowest standard NC curve (NC-15 to NC-65) that is nowhere exceeded by measured octave-band sound pressure levels, per ANSI/ASA S12.2 tangent method.